How have you determined the shocks are bad?

Install 420SEL shocks in lieu of the nitrogen spheres?

How much $$ difference will be realized and what will become of the
hydraulic system?

I wonder if the 420SEL and the 560SEL use the same steel springs or
whether the 560SEL rear depends on the hydraulic system for basic
support rather than just leveling when rear passengers weigh down the back.

The hydraulic load levelers do much more that what shock absorbers do.
They actually change dependent upon the amout of weight (load) that is
in your car, keeping your car at optimum balance regardless of the
amount of weight you have in the vehicle.  If you have ever seen a
Mercedes that is "sagging" in the back, that is, the rear bumper is
obviously too close to the road and the front of the car is too high,
it is most likely because the load leveler system is defective.  There
are safety implications related to this.  Your car is designed to steer
and brake properly when the weight distribution is correct.  Less well
designed vehicles which have only shock absorbers in the rear are not
as safe as vehicles with load levelers which keep your vehicle at
optimum balance regardless of load.  Changing to shocks would make your
car unsafe to drive.

Are your spring struts leaking?  (They are not shock absorbers, and do
not absorb shock, that function is performed by a part called the
"accumulator")  The only reason to replace the spring struts is if they
are leaking.  My 1982 TDT has 394,000 miles on it and I have replaced
both rear struts twice, the first time with used and the second time
with new parts.

I have done the same with the accumulators.  These parts are readily
available online.  If you are experiencing rough ride but your struts
are not leaking your accumulators have probably failed.  They are a
metal sphere with a rubber diaphram in the middle inside separating an
air chamber from a chamber that is part of the hydraulic fluid path of
the leveling system.  When you hit a bump and pressure increases in the
system, the rubber diaphram stretches and compresses the air in the air
chamber, absorbing the energy of the deflection of the axle because of
the bump.  When the diaphrams fail the air space fills with hydraulic
fluid.  A property of fluids is that they do not compress, so there is
no more cushioning effect in the system and your rear end starts to
bounce.  That might be good in bed but it is terrible for driving a
car.

There is also a very expensive valve on the system that is what
controls the flow of fluid based upon the deflection of a torsion bar
that is part of the rear suspension.  You hope that has not failed
because they cost about $500.  You should change the fluid and filter
in the system annually to prevent damage to the parts and make sure
that the fluid level is maintained or you will have to replace the
entire system, which is how I learned about it the hard way shortly
after buying my car in 1993.  Cost in 1995 was $1200 U.S.

I forgot to mention that you don't need to use the expensive hydraulic
fluid that Mercedes sells for about $20 a liter.  You can use anything
that lubricates and flows through a tube.  I have tried motor oil and
transmission fluid.  If you have slow leaks with the hydraulic fluid in
your struts, try motor oil, it likely will work for about 200,000 miles
before the leaks get bad.

If your struts don't leak, use automatic transmission fluid.  I use the
Chevron stuff they sell at Costco.  It's cheap, it doesn't foam badly
or leak, and I have about 280,000 miles on my valve and struts with no
problems.

I decided to try it after a crazy friend of mine was caught up at his
summer home in the mountains of eastern Washington with no brake fluid
for his Cadillac after he forgot to tighten a bleeder and the best
thing he could find (or at least that'w what he thought) was paint and
he made it back to Seattle with paint for brake fluid.

I just posted a long message about the levelers (rear shocks) on the
message board but I forgot to send it to you directly, so check the
board again.

I figured if he could run paint I ought to be able to get away with
transmission fluid and it has worked for hundreds of thousands of miles
in my car with no ill effects noticeable.